Electrolytic process for treating a ferrous sulphate solution



Nov. 27, 1945. E, A. SCHUMACHER ETAL 2,389,691

ELECTROLYTIC PROCESS FOR TREATING A FERROUS SULPHATE SOLUTION Filed Nov. 8, 1941 FERR/C FER SULFATE SULFAT MERCURY Pmni d Nov. 27, 1945 ELECTROLYTIC PROCESS FOR TREATING A FERROUS SULPHATE SOLUTION Erwin A. Schumacher, Parma, and George W. Heise, Rocky River, Ohio, assignors to National Carbon Company, Inc., a corporation of New York Application November 8, 1941, Serial No. 418,376

1 Claim.

The steel industry produces very large quantities of used acid pickle liquor, the disposal of which presents vexatious problems. Used pickle liquor contains ferrous salt and excess acid, usually ferrous sulphate and sulphuric acid. It is possible to recover the iron salts and the residual acid, and to concentrate the latter for reuse, but the market for ferrous sulphate is so limited that it can not absorb at an attractive price the product of waste pickle liquor of the entire steel industry. Laws designed to prevent pollution of streams and ground waters limit or prohibit in many places the discarding of spent pickle liquor even after it has been neutralized with lime. Thus, the disposal of spent liquor often costs as much as, or more than, the original pickle solution.

It has been proposed to oxidize the ferrous sulphate of waste pickle liquor to ferric sulphate, and to dry and decompose the ferric salt to iron and sulphur trioxide, thereby recovering in usable form substantially all of the sulphuric acid of the original pickle liquor. The principal problem to be solved in reducing this proposal to practical operation is to oxidize the ferrous sulphate cheaply and efficiently in strongly acid solution.

Anodic oxidation in an electrolytic cell, with the simultaneous deposition of iron offers a way to convert ferrous sulphate to ferric sulphate. However, there are two outstanding difliculties in such a method. Iron, being more anodic than hydrogen, can not normally be deposited from an electrolyte as acid as is ordinary waste pickle liquor. Further, if the ferric sulphate produced at the anode reaches the cathode any deposited iron is redissolved.

The object of this invention is to provide a novel continuous process for electrolytically treating an acidulou aqueous solution of a ferrous salt, for example a ferrous sulphate waste iron-pickle liquor, to produce a ferric salt and concurrently to electrodeposit iron in any desired form.

The process of this invention is practiced in a two-compartment cell, the compartments being interconnected solely by a circulating body of mercury, for instance as in the Castner cell used for making caustic soda; a porous carbon anode in one compartment of the cell; and a suitable cathode to receive deposited iron in the other compartment. In the first-mentioned compartment, acidulous ferrous sulph'ate or other ferrous salt solution is electrolyzed; the anodically oxidized trivalent iron salt is withdrawn, as soon as it is formed, through the porous anode; and

iron is. deposited at the mercury which has a high hydrogen overvoltage and acts eifectively as the cathode of that cell. The mercury, containing iron, is passed into the second compartment in which it serves as an anode. In the second compartment, there is maintained a suitable cathode and an electrolyte of a composition well suited to the deposition of iron as a smooth, coherent, solid sheet or, if desired, as a loosely adherent powder.

The sole figure of the accompanying drawing illustrates diagrammatically an apparatus suitable for use in practicing the invention. Inthe figure, there is shown in section a cell container 10 provided with an electrically insulating partition H which separates the container 10 into two compartments A and B which interconnect through one or more apertures at the bottom of the partition I l. A body of mercury rests on the bottom of the container Ill and is deep enough to cover the top of the aperture in the partition ll during operation of the cell. Spent pickle liquor, or other acidulous aqueous solution of ferrous sulphate, is fed into a first compartment A in which there is maintained a porous carbon anode l3. An electrolyte H of a desired composition is maintained in a second compartment B, and in the latter electrolyte there is suspended a cathode IS on whichiron is deposited. The body of mercury l2 serves as the cathode of the first compartment A and also as the anode of the second compartment B. A cam l6 and a pivot IT, for which there may be substituted'any suitable rocking means, are provided to rock the body of mercury l2 to force a circulation thereof between the compartments A and B and to accelerate distribution of iron in the mercury.

Electrolysi of the spent pickle liquor in the first compartment A deposits iron at the mercury cathode l2 and oxidizes ferrous sulphate to ferric sulphate at the anode [3. By promptly withdrawing anolyte through the porous carbon anode 13 as fast as ferric sulphate is formed, migration of ferric sulphate to the cathode is avoided. The rate of withdrawal of anolyte through the anode may be so adjusted that very little ferrous sulphate is withdrawn yet substantially all of the ferric sulphate formed is recovered.

The porous carbon anode l3 may be constructed as described in Porous carbon electrodesby George W. Heise, Transactions of The Electrochemical Society, vol. 75, pages 147 to 166, 1939.

Mercury containing iron deposited in the first compartment A is circulated under the intercompartment partition ll into the second compartment B where the iron is removed by anodlc action into the electrolyte II and thence deposited on the cathode l which may suitably be made of iron.

Other and less simple, though mechanically more efllcient, means for circulating the mercury I2 may of course be used instead of rocking. For instance, the two cell compartments A and B may be in separate containers interconnected by pipes through which the mercury is circulated by pumps or stirrers. Circulation may be improved by imposing a rotating magnetic field or by other known expedients.

Liquors having an excess acid content up to about 2% or 3% by weight can be handled efficiently in the manner just described. With some sacrifice in efiiciency an acidity as high as 4% or 5% can be tolerated. Much of the spent pickle liquor contains from 0.5% to 3%- of free acid, and by mixing such acid with the more acid liquors, containing say 7% free acid, obtained from some pickling operations, it will readily be possible to treat the entire output of spent pickle liquor of nearly any steel producing locality.

The composition of the electrolyte in the second compartment B may be adjusted for optimum plating conditions. Successful results may be obtained with an electrolyte containing about 350 gm./liter of ferrous ammonium sulphate kept in a slightly acid condition, e. g. 0.01 N to 0.02 N sulphuric acid; but it is feasible to depart from this range of compositions without departing from the invention.

Although this invention was developed primarily to treat ferrous sulphate solutions, it will be apparent that other aqueous ferrous salt solutions, for instance a ferrous chloride solution, can

be treated. Similarly, salts of other multi-valent metals, such as stannous tin or cuprous copper, may be substituted for the iron salts. Nor is it at all necessary that the same salt of iron be used in both of the compartments A and B.

Furthermore, porous conductors other than carbon, for instance porous metal conductors, may be substituted for the porous carbon anode. Likewise, it will sometimes be desirable to remove the iron from mercury cathode of compartment A in a physically separate cell, or even to dissolve out the iron by nonelectrolytic chemical methods. Such modifications are deemed to be within the invention.

We claim:

A continuous process for treating an acidulous aqueous solution of ferrous sulphate containing 0.5% to 5% free sulphuric acid to produce iron and a solution of ferric sulphate which comprises the steps of electrolyzing such solution as an electrolyte in contact with a mercury cathode in which iron is deposited, and a porous carbon anode at which ferrous sulphate is oxidized to ferric sulphate; withdrawing through said anode 

